Plastic Ball Grid Array (PBGA) chip carriers, such as the Over Molded Pad Array Carrier (OMPAC.TM.), the Over Molded Peripheral Chip Carrier (OMPCC), and the Over Molded Pin Grid Array (OMPGA) and their equivalent glob top printed circuit board chip carriers are susceptible to moisture absorption. These integrated circuit packages suffer from a well-known phenomena known as "popcorning" when subjected to heat. This problem is particularly noticeable in surface mount packages using printed circuit board (PCB) substrates, which experience thermal and mechanical stresses due to the exposure of the entire package to solder reflow temperatures. Typically, these integrated circuit packages are baked prior to solder assembly to remove moisture. Moisture usually penetrates into and through the molded plastic resin encapsulant, the PCB substrate, and especially the adhesive used to attach the die to the substrate. The die attach adhesive is usually a hygroscopic material. When the package is exposed to solder reflow temperatures after the die attach adhesive has absorbed moisture, the moisture is rapidly converted into a gas, expanding dramatically. This causes the die to delaminate from the substrate, creating the "popcorn" effect.
U.S. Pat. No. 4,866,506 by Nambu et. al. discusses a flat plastic-sealed lead frame package that has an opening on an underside of the package, allowing the die flag to be exposed to the atmosphere, thereby venting or releasing of moisture when subjected to heat. This package, also known as the Quad Flat Pack (QFP), is easily manufactured, but leaves the bottom surface of the lead frame exposed to the atmosphere. The QFP does not use a substrate or printed circuit board as in OMPAC, OMPCC, and OMPGA and their glob top equivalents; thus, a different solution is required.
U.S. Pat. No. 5,296,738 by Freyman et. al., discusses a printed circuit board chip carrier with an opening(s) in the PCB substrate, located directly beneath the semiconductor die, which provides the means for moisture relief when the package is subjected to extreme temperature excursions (i.e., board reflow profiles). In this package, the opening is initially covered on the back side of the printed circuit board substrate with solder resist. The covered opening prevents the flow of die attach material from the top side surface through the opening and onto the backside solder pads; however, the opening(s) are partially or completely filled with die attach material. After final assembly of the package, the solder resist must be pierced or punctured to create the final opening for moisture relief.